As of this writing, VXX currently has its holdings divided between the January (F4) and February (G4) 2014 VIX futures contracts. As the January contract nears expiration, the folks at iPath will have to replace it with the March contract (H4), and presumably at a higher price. That price premium will then decay back down toward where the spot VIX is as the contract nears expiration. So VXX shareholders are continuously being victimized by the "roll" to later expiration month contracts, with that ETN buying higher and then selling lower, and repeating. That explains why the VXX's long term "performance" has been so awful.